1. Field of the Invention
The present invention relates to an optical coupling element and an optical device, more specifically an optical coupling element which can effectively prevent residual reflection without sacrificing optical coupling efficiency, and an optical device comprising the optical coupling element.
2. Description of the Related Art
It is known that internal residual reflections of optical power in optical devices are a factor degrading device performance. For example, in laser oscillators, when residual reflection light returns into the oscillator, the residual reflection light disturbs oscillation states, which is a factor increasing noise components. A method to suppress the generation of the residual reflection light in the optical coupling elements is very important.
Usually the most important source of reflections is the device structure to couple optical power in and out of the chip on which the optical device or the optical integrated circuit is located. These structures have regions where the optical devices contacts the atmosphere, and due to a large refractive index difference between material forming the optical device and air, optical power is reflected at the interface therebetween to become residual reflection light.
Conventionally, methods for preventing the residual reflection, the following methods, for example, are proposed.
In a first method, an anti-reflection film is formed on the chip facet. This method can reduce the reflectivity of the chip facet.
In a second method, the optical waveguide is tilted by a prescribed angle (e.g., about 7°) relative to a normal vector on the chip facet. This method can decrease the amount of light reflected at the chip facet and coupled back into the optical waveguide.
In a third method, the width of the optical waveguide is tapered toward the output end. This method widens the effective optical field distribution at the chip facet, whereby the amount of light reflected at the chip facet and coupled back into the optical waveguide can be decreased, and also the optical field-width of the optical power can be easily matched with the optical field-width of the optical propagation element optically coupled to the output end of the optical waveguide.
In a fourth method, a so-called window structure is used. A window structure is a structure in which the end of the optical waveguide is spaced from the chip facet. In cases using the window structure, the optical field width of the optical power outputted from the end of the optical waveguide increases gradually in the direction of propagation of the optical power. Accordingly, this method can reduce the amount of light reflected at the chip facet and coupled back into the optical waveguide can be decreased.
These methods are used separately, or in combinations of two or more.
FIG. 9A is a top view of the structure of an optical coupling element using the second to the fourth methods described above. That is, in the optical coupling element of FIG. 9A, the optical waveguide 100 is tapered so as to have a smaller width at the output end surface 104 than at the input end surface 102, a window region is provided between the output end surface 104 and the chip facet 110, and the optical waveguide 100 is arranged so that a propagation direction of the optical power on the chip has a prescribed tilt angle to a normal vector on the chip facet 110. The optical coupling element of FIG. 9A can effectively prevent reflection light, reflected at the chip facet 110, from returning into the optical waveguide 100.
On the other hand, in the optical coupling element, it is very important not only to prevent residual reflection near the chip facet but also increasing optical coupling efficiency between the optical waveguide formed on the chip and external optical element, such as optical fibers, etc. formed outside the chip. To this end, an end configuration of the end of the optical waveguide, e.g., the length of the tapered region and the width of the end surface are optimized by simulation tools so as to obtain good optical coupling efficiency.
The optical coupling element shown in FIG. 9A has a structure which is very effective to prevent reflection light from the chip facet 110 from returning into the optical waveguide 100. However, it cannot be said that the structure is sufficient for suppressing residual reflection. That is, as shown in FIG. 9B, reflection light is generated at the output end surface of the optical waveguide 100 inside the chip, but the optical coupling element shown in FIG. 9A does not address the reflection at the output end surface 104 of the optical waveguide 100 and cannot suppress the generation of the residual reflection light due to this reflection at the output end surface 104.
In order to suppress the reflection at the output end surface of the optical waveguide, it is considered to taper the optical waveguide so that the optical waveguide has a zero-width at the end surface of the optical waveguide. However, an optimum value of the optical coupling efficiency can be obtained in a tapered structure, in which the end surface has a prescribed non-zero width. Accordingly, the tapered structure, in which the end surface of an optical waveguide has a zero-width, has lower optical coupling efficiency in comparison with the tapered structure, in which the end surface has a prescribed non-zero width. In terms of fabrication, it is very difficult to form with good reproducibility the optical waveguide having a zero-width end surface and the end thereof accurately aligned.